MPR084

Sensors

Freescale Semiconductor

17

Preliminary

5

Low Power Configuration

5.1

Introduction

The MPR084 features a Low Power mode that can reduce the power consumption into the microamps range. This feature can

be used to both adjust the response time of the system, and change the conditions on which Low Power would be enabled.

5.2

Operation

This Low Power configuration is only active when the sensor controller is in Run2 mode. The Low Power mode decreases current

consumption by increasing the response time of the MPR084. This increase is controlled through two factors.

During normal Run2 operation of the sensor controller the Max Response Time (MRT) is calculated by taking the product of the

TASP and the primary clock. From Chapter 4 the primary clock is the (MTP + 5) ms. Since the sensor controller is in Run2, the

primary clock is also multiplied by a factor of 8. The debounce rate of the MPR084 is 4 times the sample rate thus the MRT is

represented by the following equation.

Equation 1

First, the Idle Interface Timeout (IIT) represents the total time the touch interface should remain idle before going into Low Power

mode. This value can be calculated by taking the product of the ITP, TASP and primary clock (8ms) with a factor of 64. Thus the

IIT is represented as follows:

Equation 2

Second, the Max Response Time (MRT) represents the total time the touch interface should remain inactive before scanning the

electrodes. This value can be calculated by taking the product of the SCD, TASP and primary clock (8ms) with a factor of 5. Thus

the MRT is represented as follows:

Equation 3

Figure 20. Low Power Scan Period Transition Diagram

5.3

Configuration

Low Power Configuration is achieved through setting two values; the Idle Timeout Period and the Sleep Cycle Duration. This

functionality is described in the following section.

MTP

5

+

8

----------------------

1

+

⎝⎠

⎛⎞

TASP

48ms

×

×

×

=

MTP

5

+

8

----------------------

1

+

⎝⎠

⎛⎞

TASP

SCD

48

× ms

×

×

×

=

ITT

MTP

5

+

8

----------------------

1

+

⎝⎠

⎛⎞

TASP

ITP

×

×

68ms

×

×

=

LP DISABLED

ITT PERIOD

run2 SET

TOUCH DETECTED

MRT1

MRT2